In a typical nuclear reactor, such as a pressurized water (PWR), heavy water or a boiling water reactor (BWR), the reactor core includes a large number of fuel assemblies, each of which is composed of a plurality of elongated fuel elements or rods. The fuel rods each contain fissile material such as uranium dioxide (UO2) or plutonium dioxide (PUO2), or mixtures of these, usually in the form of a stack of nuclear fuel pellets, although annular or particle forms of fuel are also used. The fuel rods are grouped together in an array which is organized to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A coolant, such as water, is pumped through the core in order to extract some of the heat generated in the core for the production of useful work. Fuel assemblies vary in size and design depending on the desired size of the core and the size of the reactor.
When a new reactor starts, its core is often divided into a plurality, e.g. three or more groups of assemblies which can be distinguished by their position in the core and/or their enrichment level. For example, a first batch or region may be enriched to an isotopic content of 2.0% uranium-235. A second batch or region may be enriched to 2.5% uranium-235, and a third batch may be enriched to 3.5% uranium-235. After about 10–24 months of operation, the reactor is typically shut down and the first fuel batch is removed and replaced by a new batch, usually of a higher level of enrichment (up to a preferred maximum level of enrichment). Subsequent cycles repeat this sequence at intervals in the range of from about 8–24 months. Refueling as described above is required because the reactor can operate as a nuclear device only so long as it remains a critical mass. Thus, nuclear reactors are provided with sufficient excess reactivity at the beginning of a fuel cycle to allow operation for a specified time period, usually between about six to eighteen months.
Since a reactor operates only slightly supercritical, the excess reactivity supplied at the beginning of a cycle must be counteracted. Various methods to counteract the initial excess reactivity have been devised, including insertion of control rods in the reactor core and the addition of neutron absorbing elements to the fuel. Such neutron absorbers, known in the art and referred to herein as “burnable poisons” or “burnable absorbers”, include, for example, boron, gadolinium, cadmium, samarium, erbium and europium compounds. Burnable poisons absorb the initial excess amount of neutrons while (in the best case) producing no new or additional neutrons or changing into new poisons as a result of neutron absorption. During the early stages of operation of such a fuel element, excess neutrons are absorbed by the burnable poison, which preferably undergoes transformation to elements of low neutron cross section, which do not substantially affect the reactivity of the fuel element in the later period of its life when the neutron availability is lower.
Sintered pellets of nuclear fuel having an admixture of a boron-containing compound or other burnable poison are known. See, for example, U.S. Pat. Nos. 3,349,152; 3,520,958; and 4,774,051. However, nuclear fuel pellets containing an admixture of a boron burnable absorber with the fuel have not been used in large land-based reactors due to concerns that boron would react with the fuel, and because the use of boron was thought to create high internal rod pressurization from the accumulation of helium in the reaction:10B+1n→11B(excited state)→4He+7Li
Current practice is to coat the surface of the pellets with a boron-containing compound such as ZrB2, which avoids any potential reaction with the fuel. However, this does not solve the pressurization problem, which limits the amount of coating that can be contained within each rod. More rods with a lower 10B loading must be used, thus necessitating the handling and coating of a large number of fuel pellets, which is very expensive and results in high overhead costs. Complex manufacturing operations also result from the need to separate the coated and non-coated fuel manufacturing and assembly operations. In practice, the cost of coating the pellets limits their use, and they are used in as few rods as possible, taking into account the pressurization problem described above. Historically this was acceptable, because fuel cycles were shorter, levels of 235U enrichment were lower, and overall thermal output of a reactor was lower.
Other compounds such as Gd2O3 and Er2O3 can be added directly to the pellets, but these are less preferred than boron because they leave a long-lived, high cross-section residual reactive material.
Nuclear reactor core configurations having burnable poisons have been described in the art. For example, U.S. Pat. No. 5,075,075 discloses a nuclear reactor core having a first group of rods containing fissionable material and no burnable absorber and a second group of rods containing fissionable material with a burnable absorber, wherein the number of rods in the first group is larger than the number of rods in the second group. The burnable absorber comprises a combination of an erbium compound and a boron compound.
U.S. Pat. No. 5,337,337 discloses a fuel assembly where fuel rods containing a burnable poison element having a smaller neutron absorption cross-section (such as boron) are placed in a region of the core having soft neutron energy and a large thermal neutron flux, while rods having a burnable poison element having a larger neutron absorption cross-section (such as gadolinium) are placed in regions of the core having average neutron energy spectrum. Neither of these prior patents disclose an arrangement of fuel rods in fuel assmeblies in which a majority of fuel rods contain boron alone, as the burnable poison. Neither disclose assembly arrangements suitable for reactors producing over 500 megawatts thermal power.
With the use of longer fuel cycles and higher levels of 235U enrichment, there remains a need for the development of nuclear fuels and fuel assemblies having integral burnable absorbers that are cost-effective and can extend the life of the fuel without creating additional reactive materials.